Before you ever take a student up on a buddy box, take an accomplished flier up first ... ask him to fly around and unexpectedly "make mistakes" that you need to take control over ... this accomplishes at least 2 things:

1) let's you practice dealing with the buddy-box switch (and some of them - especially Futaba - are a real pain to use) in such a way that if you screw up and don't take control back in time, the guy on the slave box is accomlished enough a pilot that he'll fix his flying problem and not fly into the ground

2) Gives you feedback about whether you are being too quick to take control back, or to slow, or just about right